Spectrally tuned solar cells for improved energy harvesting

The PV Tune project is an applied research project funded by the Cyprus Research Promotion Foundation. The project consortium is developing a methodology to help photovoltaic cell manufacturers optimise their designs by predicting how the spectral response of cells affects their annual energy yield.

What's it about?

The transition from finite, fossil-fuel based energy sources to clean, renewable energy sources is one of the great technical challenges facing mankind today.

The sun is an abundant, renewable and widely-distributed energy source that could easily provide all the energy required by our modern lifestyles.

Photovoltaic power stations convert sunlight directly into electricity, and are now a well-established renewable energy technology providing clean, affordable energy to millions of people across the world.

At the University of Cyprus we are studying the electricity generation potential for the next generation of photovoltaic technologies that are even more efficient than today's systems.

These next-generation devices work by using a greater proportion of sunlight reaching them, resulting in an overall increase in energy conversion efficiencies.

A new approach to designing advanced solar cells means they can be optimised for the sky conditions at a given location, potentially resulting in even higher energy yields.

PV-TUNE is a project that aims to quantify the potential for increasing solar energy yields through optimisation of photovoltaic cell designs. The project is a collaboration between a group of leading institutions in the field of high-concentration photovoltaics with the goal of collecting real outdoor data to support the theoretical gains in performance suggested by models.

The PV-TUNE project started in June 2013, and is scheduled to run for 24 months.

Project Overview

Scientific Objectives

Generation of a spectral resource dataset for a 'Typical Meteorological Year' in Cyprus.

A key aspect of evaluating the potential for a new cell design is to know the spectral irradiance resource at the location in which it will be used. For this reason, a dataset will be prepared that will be representative of the spectral resource of a 'Typical Meteorological Year' in Nicosia, Cyprus. This will be prepared using the SMARTS atmospheric model and validated using real on-site measurements.

Creation and validation of an energy production model for triple-junction cells under real operating conditions.

Given the spectral irradiance resource of an installation site, in combination with other environmental parameters, a model will be prepared to simulate the output of a module incorporating multi-junction photovoltaic cells. This model will include all the known phenomena occurring within the cell including coupling effects between cells, and will be used to evaluate the suitability of a particular cell design for a given location.

Development of standardised practices for multi-junction PV cell measurement.

To understand the performance of multi-junction cells under real operating conditions, a comprehensive set of characterisation procedures are required. These are to be performed indoors under controlled conditions, and cells are to be exposed to different temperatures and irradiance conditions. This work will support efforts to develop international standards for the characterisation procedures of multi-junction cells, as well as feed information directly into the models being developed within the PV-TUNE project.

At present, the IEC technical committee 82, working group 7 is developing international standards for the energy rating procedures for concentrator photovoltaic modules. These standards are still at an early stage of development, and the work being performed within the scope of the PV-TUNE project will feed directly into developing better understanding of the uncertainties and challenges of assigning energy ratings to this technology.

The understanding of the performance of CPV modules under outdoor conditions relies on a detailed understanding of the performance characteristics of the cells within the modules under a range of different operating conditions. Therefore the PV-TUNE project will involve a detailed assessment of the indoor cell test procedure uncertainties in order to improve the confidence in outdoor measurements and to correct observations to standard reporting conditions.

Technical Objectives

The acquisition of a comprehensive environmental dataset with low measurement uncertainty.

One of the cornerstones of the analyses to be performed within the scope of the PV-TUNE project is a high-quality set of environmental parameters measured regularly, with minimal disturbances, and to a traceable level of uncertainty. For this reason, great care has been taken to prepare a high-quality meteorological measurement station, which includes:

Wind speed and direction

Ambient Temperature

Direct Normal and Global Normal broadband solar irradiance

Air pressure and humidity levels

Spectral irradiance measurements over the range 300−1700 nm

All these parameters are recorded at 5 minute intervals, and traceable to international reference centres where possible.

Creation of a high-quality infrastructure for spectrally-resolved measurement of the solar irradiation.

Due to the high costs of the required equipment, there are few locations across the world where high-resolution measurements of the direct normal solar spectrum is being recorded. Furthermore, the effort to maintain a reliable measurement infrastructure over extended periods of time make it harder to find high-quality spectrally-resolved DNI measurements. One outcome of the PV-TUNE project will be just such a dataset that will be made available for analysis and use in validation of models.

Fabrication of a set of robust, high-efficiency concentrator photovoltaic (CPV) modules for continuous long-term outdoor operation.

To investigate the effect of spectral response on energy yield over the period of a year, two sets of small CPV modules will be fabricated that will each contain a different type of photovoltaic cells with a correspondingly different spectral response. To determine the difference in output due to the cell design, the modules will be fabricated in an identical way and continually monitored side-by-side outdoors under real operating conditions. A year of data will be collected alongside the environmental dataset described above, and this will be used to extract information that will indicate how the spectral response of the cells affects the final energy yield of each cell design.

Development of an advanced characterisation set-up for multi-junction cell measurement.

To perform the advanced measurements required for a detailed understanding of the cell performance under real operating conditions, a state-of-the-art infrastructure will be assembled for indoor characterisation of multi-junction cells. The set-up will allow the measurement of device spectral response, spectrally-resolved electroluminescence, photoluminescence, temperature coefficients over the anticipated outdoor operating ranges as well as allowing the study of advanced intra-junction effects such as luminescent coupling.

Round-robin measurement campaign of MJ cells and CPV modules.

In order to lower the inherent measurement uncertainties and to improve confidence levels in the measurements being taken on site, a number of leading institutions in the field of CPV module and cell measurement will be involved in the evaluation of the module performances at different sites. The comparison of current-voltage characteristics will provide valuable information on the quality of the monitoring infrastructures involved.

Modelling and validation of the spectrally-resolved solar resource in Nicosia, Cyprus using the SMARTS atmospheric modelling tool.

The information gained through the environmental monitoring campaign will be fed into the SMARTS atmospheric model to produce a year's dataset of 5-minute direct normal spectral irradiance values. The data will be filtered to remove occasions where the irradiance levels are too low to significantly contribute to the yield of CPV systems, and thereby deal with clear-sky conditions that are suitable for electricity generation. The results will be validated against the measured spectral conditions corresponding to the on-site environmental conditions, and the model refined until the closest possible agreement is obtained. Finally, a 'Typical Meteorological Year' dataset will be fed into the model to arrive at a corresponding 'Typical Annual Spectra Resource'.

Project Partners

The PV-TUNE project is co-financed by the European Regional Development Fund and the Republic of Cyprus through the Research Promotion Foundation (Cyprus) in the framework of the project ‘Spectrally Tuned Solar Cells for Improved Energy Harvesting’ with grant number ΤΕΧΝΟΛΟΓΙΑ/ΕΝΕΡΓ/0311(ΒΙΕ)/13.

Proceedings of the 28th European Photovoltaic and Solar Energy Conference and Exhibition, Paris, France, September 2013, WIP Munich.

Spectral measurements are an important aspect of environmental monitoring for photovoltaics, especially concentrator photovoltaics (CPV) that use multi-junction cells with spectrally dependent output. There are only a few reliable sources of spectrally resolved direct normal irradiance (DNI) data in the world since the measurement equipment is expensive and calibration is hard to maintain. Efforts have been made to produce a high quality data set of spectra with reduced measurement uncertainties. An evaluation of the uncertainty of both indoor and outdoor spectrum measurements and their origins is presented in the paper. A traceable measurement system was used to calibrate two sets of spectroradiometers and repeated measurements have been carried out to find the uncertainties of measurement repeatability, positional sensitivity and lamp stability. The two measurement systems were then used outdoors alongside broadband solar irradiance measurement devices, to determine the differences of the spectral measurements between the systems. Finally, a collimator was attached to one of the spectroradiometer systems and measurements were taken using both collimated and non-collimated systems to establish the impact of the collimator on spectral measurements.

Sol. Energy Mater. Sol. Cells vol. 130, pp. 170–181, 2014

In multi-junction devices, due to the series connection of junctions, recombination current from the top junctions can be directed to the bottom ones affecting their electrical characteristics. Recently, luminescence coupling effects during External Quantum Efficiency (EQE) measurements at very intense light bias conditions indicated high recombination current flowing towards the bottom junctions of the cells. In an attempt to find the magnitude of coupling current as well as the factors affecting the optical interactions between junctions, excitation and voltage dependent Photoluminescence (PL) measure- ments of GaInP/GaInAs/Ge have been carried out. An investigation using junctions with different shunt resistances has been conducted to identify the impact of shunts on the coupling current. Furthermore the impact of temperature on the coupling current has been considered. Our results show that a maximum of 2.3% of the recombination current of the top junction is converted to coupling current in the middle junction depending on the devices under examination. The coupling efficiency depends on the shunt resistance of the top junctions as well as on the temperature. Furthermore a physical model of the current limiting junction was built taking into consideration the impact of local ohmic shunts in the solar cell device and used to validate the experimental data taken.

V.Paraskeva, M.Norton, M.Hadjipanayi, M. Pravettoni, G.E.Georghiou

Sol. Energy Mater. Sol. Cells vol. 134, pp. 175–184, 2015

Combined electroluminescence (EL) and photoluminescence (PL) measurements were conducted in order to investigate the presence of luminescent emission of InGaP/InGaAs/Ge at different operating conditions of the tandem. Luminescent emission from cell samples was observed at different sun concentrations, voltage biases and temperatures. A high intensity pulsed solar simulator was used to photoexcite the device which exhibited strong radiative recombination from both the top InGaP and middle InGaAs junctions. Luminescent emission from the device was investigated under a range of voltage biases and was clearly observed at the maximum power point voltage of the sample under test indicating its presence during typical operating conditions of the solar cells. Investigation of the emission was also performed at relatively high temperatures (up to 60 1C) in order to mimic the outdoor operating conditions of a solar cell device. Luminescence was detected at high temperatures indicating that significant radiative recombination is present at even higher temperatures. Outdoor measurements under actual solar spectrum demonstrated the presence of luminescent emission in agreement with indoor testing. The significant amount of radiative recombination at the band-gap edges of the top junctions observed in our measurements gives evidence that optical coupling to the lower ones may occur. Finally, excitation power dependent PL was performed using monochromatic laser sources in order to investigate the impact of externally induced photocurrents of different intensity upon the radiative signal of each junction.

V.Paraskeva, M.Hadjipanayi, M.Norton, M.Pravettoni, G.E.Georghiou

Series connection of multi-junction devices can lead to opto-electronic interactions between junctions and thus coupling effects. These effects can be important during External Quantum Efficiency (EQE) measurements of multi-junction devices. In an attempt to find the impact of coupling effects on different shunt resistance devices, EQE measurements have been carried out at high intensity light bias conditions. These measurements showed that in those conditions, the coupling current in high quality materials is considerably higher compared to low quality ones and lead to a higher reduction of the EQE signal. The difference in EQE is, nevertheless, small and it is apparent in all the response region of the material.

M.Norton, V.Paraskeva, R.Galleano, G.Makrides, R.Kenny, G.E.Georghiou

Proc.29th EUPVSEC (2014) pp.2002-2007

Photovoltaic cells that incorporate several active junctions in electrical series are demonstrating ever- higher laboratory efficiencies. However, their sensitivity to variations in the solar spectrum can reduce their operating efficiency in the field. To assess how these variations can affect energy yield requires a detailed and reliable spectral irradiance dataset covering a typical meteorological year for any potential deployment site. This paper describes an attempt to produce such a dataset through a combination of measurements and simulation. A dedicated measurement system has been assembled to collect environmental data alongside direct normal spectral irradiance measurements utilising charge-coupled device array spectroradiometers. The calibration and validation procedures for this system are presented, and the final calibration uncertainty of the system is judged to be ±5.4% (k=2). The SMARTS atmospheric radiative transfer model is used to provide another means of measurement validation. Furthermore, SMARTS is then used to generate long-term irradiance data covering several months. A comparison of the measurements and simulations has shown that SMARTS is a useful tool for measurement validation. However, the present environmental inputs to the model produce long-term data that underestimates the occurrence of very blue- rich spectra compared to measurements.

N. Kindyni, C. Lazarou, George E. Georghiou

Proc. 40th IEEE Photovoltaic Specialist Conference (2014)

The authors originally described potential-induced degradation (PID) as a sequence of interdependent mechanisms based on the physics behind the aging behavior of an NMOS: with low-k dielectric in the sub-100 nm thickness range, under conditions of high temperature and electric field, with mobile ion contamination and positive bias on the metal electrode. This paper reports the development and application of a two dimensional time dependent numerical model for conventional silicon solar cells for the study of the effect of positive ion deposition on the cell’s surface occurring during PID. The results obtained from the numerical model show good initial agreement with the proposed PID theory.

M.Norton, V.Paraskeva, R.Kenny, G.E.Georghiou

Proc. 42nd IEEE Photovoltaic Specialist Conference (2015)(Preprint)

The operating efficiencies of multi-junction photo- voltaic cells are sensitive to changes in the spectral distribution of solar irradiance. To examine how this affects their performance in the field, two sets of triple-junction photovoltaic cells with different spectral responses are being characterised side-by-side outdoors. Initial analyses of the measurements have revealed seasonal differences between the performances of the cells that can be attributed to their spectral responses. This suggests that more long-term data will show how the choice of spectral response can impact the annual performance ratios for multi- junction photovoltaic systems.

Contact

If you wish to have further information on the work carried out in this project, or are interested in possible collaboration, please feel free to email us at the following address:matthewn@ucy.ac.cyDr Matthew Norton

Our site is located at the University of Cyprus' Photovoltaic Technology Park, in Nicosia Cyprus. The coordinates of this location are:‎
35.147705, 33.416977‎
+35° 8' 51.74", +33° 25' 1.12"See it in Google Maps